Two-stroke cycle, or two cycle, engines are known to suffer combustion process deterioration at part load. To understand this phenomena, it is helpful to contrast two-stroke cycle engine scavenging with four-stroke cycle engine scavenging. In the two cycle engine, the scavenge air enters the cylinder while the exhaust port is open. This means that cylinder pressure is nearly atmospheric and the entering scavenge air is thus at nearly atmospheric pressure and temperature. The resultant high density of the scavenge air means its volume is small at the small delivery ratios characteristic of part load operation. As a result, the small volume of scavenge air is unable to purge the cylinder of much exhaust gas residual. In contrast, in the four cycle engine the entire cylinder displacement volume is purged of exhaust residual gases by the piston. As a result, the incoming fresh charge, which is at low density corresponding roughly to manifold vacuum, fills most of the cylinder volume. The result of this is that the four cycle combustion process is more stable and reliable because the mass fraction of residual in the cylinder is smaller at a given load than that of a two-stroke engine. In addition, the mixture in the vicinity of the spark plug at the time of ignition in a four cycle engine is more likely to be low in residual than for the two cycle. In summary, one of the principal differences in operation of two cycle and four cycle engines at part load is that the four cycle charge is at lower density than that of the two cycle leading to less difficulty in achieving satisfactory combustion. This invention provides a means of lowering the charge density of the scavenge gases at part load to promote scavenging a greater volume of exhaust residual gases for a given delivery ratio, thus providing a combustible charge with lower mass fraction residual, greater fresh charge volume, and therefore greater likelihood of satisfactory ignition and combustion.